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Solar Toilet Disinfects Waste, Makes Hydrogen Fuel


You might take that sound for granted. I know I do. That's because most of us hear it all the time, at least in this country. Toilets are everywhere here in the U.S. But a lot of people around the world don't hear that sound every day, because two-and-a-half billion people, with the B, don't have a safe, sanitary place to go to the bathroom, according to the World Health Organization.

That's why the Gates Foundation rolled out its Reinvent the Toilet Challenge, asking engineers to dream up waterless, hygienic toilets for people in the developing world. They held a toilet fair. Yep, they held a toilet fair in Seattle this week, complete synthetic poop.

And my next guest and his team top prize with a solar-powered toilet that not only disinfects waste, but also produces hydrogen fuel. A lot better than flushing all that energy down the toilet, right? Joining me now to explain how it works is Michael Hoffmann. He's the James Irvine Professor of Environmental Sciences and Engineering at the California Institute of Technology in Pasadena, and he joins us by phone. Welcome to the show, Dr. Hoffmann.

MICHAEL HOFFMANN: Well, thank you. I'm glad to be here.

LICHTMAN: You're listening to SCIENCE FRIDAY on NPR.


LICHTMAN: I'm Flora Lichtman.


LICHTMAN: And so tell me about this toilet.

HOFFMANN: OK. This is a prototype that was displayed after one year of research. So eight major universities around the world were tasked with coming up with a new approach to completely handle the waste, basically, with no traditional urban infrastructure. In other words, it should operate in the absence of an electrical grid. It should operate in the absence of piped-in water, the subject of your previous conversations. And it should basically function at low cost and be available to the developing world within a three to four-year period of time.

LICHTMAN: And what is yours - what does your prototype look like?

HOFFMANN: OK. Right now, we take some conventional flush toilets and integrate them with a series of chemical engineering processes. That was also one of the tasks, that we should use modern approaches developed in chemical engineering to piece something together to be able to handle the waste in a single drain, basically purify the water that may be present in waste and urine. In particular, urine is a source of water.

So, in essence, we clean up the urine to a relatively high quality. That becomes the water source back into a flush tank location. And then with that water, we can then flush the actual toilet. So we have a combined urinal, which is a waterless urinal, which diverts the urine to a separate treatment process, and then that provides the water for a closed-loop cycle. In addition, the tank could be also filled with gray water from cooking or personal sanitation in the more remote regions.

LICHTMAN: And that chemical reaction is pretty fast, right? Because I think I heard that composting toilets, it takes months to disinfect the waste. But with this...


LICHTMAN: ...it's a matter of hours?

HOFFMANN: Yes. For a typical daily waste of a family - let's say we're limited to five persons. But we've actually scaled up a system for, let's say, essentially 40 uses per day. But nonetheless, let's take the waste of a single family, five people. That's about half a kilogram of feces, and then five liters of water. And that can be processed, essentially, within three to four hours.

LICHTMAN: Hmm. That's - I mean, that seems like a remarkable innovation.

HOFFMANN: Yes. Some people think it's totally impractical for application in the developing world. But we think there's a lot of promise there for the future with further developments to bring the cost of the integrated system down.

So we're essentially oxidizing the urine and waste at sort of a developed electrode system, multiple electrodes, where the reactive side, the oxidative side is composed of nanoparticulates, semiconductor materials coded in a conductive metal plate in sequence, and then basically centered onto the surface. And then the counter-electrode is simply a metal plate, a sufficiently sized metal plate. It could be almost any metal. Stainless steel is used because it's, obviously, more resistant to degradation over time.

And the electrons that are given up by the waste are essentially passed on to the counter electrode, which we call the cathode. And instead of using oxygen to accept the electrons, we use water and protons, thus generating hydrogen, which can be cleaned up a bit and then used either for cooking or directed into a proton exchange membrane fuel cell.

LICHTMAN: Wow. That is awesome. Michael Hoffmann, thank you. We've just about ran out of time.


LICHTMAN: But thank you for joining us today.

HOFFMANN: Sure. Yeah. Thank you for talking to me.

LICHTMAN: Michael Hoffmann is the James Irvine Professor of Environmental Sciences and Engineering at the California Institute of Technology in Pasadena. Transcript provided by NPR, Copyright NPR.